Respiratory Syncytial Virus (RSV) infection in the elderly causes great human suffering due to hospitalization and death, and is considered a societal burden similar to that of seasonal influenza. Vaccines for the prevention of RSV infections are not yet available, and development efforts are made all the more difficult in the elderly due to age-associated immunosenescence that impairs their ability to mount protective immune responses. A highly potent vaccine that overcomes immunosenescence is therefore sorely needed. ! RSV F subunit vaccine immunogens that hold the pre-fusion conformation (preF) induce very high neutralizing titers that correlate with potent protection. We are developing such a preF subunit immunogen, DT-preF, and animal models that will allow us to identify a DT-preF formulation that provides the elderly maximal protection from RSV. Our preliminary data suggests that our aged mouse model distinguishes between more and less potent vaccines. This proposal is further strengthened by a set of preclinical data in adult mice and cotton rats demonstrating that our DT-preF elicits high neutralizing titers, fully protects cotton rats from viral challenge, and has impressive stability in vitro. In Phase I, of this fast-track proposal, we prove that aged mice effectively distinguish between more and less potent DT-preF formulations, enabling their characterization and down-selection in immunogenicity and challenge experiments (Aim I). In Phase II, we rank and select the best two formulations for further evaluation, based on anti-F Ab and neutralization titers and cellular responses in young and aged mice immunized with eight DT-preF dose-adjuvant combinations. (Aim I). Then we select our lead dose and adjuvant pair based on viral lung titers following challenge with RSV (Aim II). We further support our mouse data in a second animal model, RSV pre-immune aged cotton rats, which better replicates human RSV infection, and that also better mirrors the RSV immune status of the elderly (Aim III). The key innovation of this proposal is our DT-preF that is formulated specifically to maximize protection in the elderly, and that uniquely overcomes immunosenescence. ! !